The projection-type display device that magnifies and displays a video is widely used from a personal theater to business presentation. As a light source for such a projection-type display device, a light source that uses an ultra high pressure mercury lamp is the mainstream. However, the light source that uses the ultra high pressure mercury lamp has problems that include short life and a significant environment effect because its uses mercury. When the ultra high pressure mercury lamp is used for the light source, an optical system becomes complex because white light from the light source is separated into lights of three primary colors, and it is difficult to be miniaturized the optical system because of the large etendue. Thus, using an ultra high pressure mercury lamp as the light source for a projection-type display that is to be miniaturized may not be the best approach.
To solve the problems, a projection-type display device that uses a semiconductor laser as a light source has been offered. Because of its high directivity, the laser light source provides advantages of high efficiency using light, low power consumption, and a long life.
Among the three primary color lights of R (red), G (green), and B (blue) used in the projection-type display device, for the red light and the blue light, high-output semiconductor lasers have been put into practical use. However, for the green light, a semiconductor laser that is durable enough to be used in display devices has not yet been developed. Thus, a SHG (Second Harmonics Generator) laser that converts infrared light (1064 nm) of a LD (laser diode) excitation solid laser or a light excitation semiconductor laser into green light (wavelength of 532 nm) by using a wavelength conversion element (SHG) has been used instead.
However, when the infrared light is converted into the green light by using the wavelength conversion element, the light source becomes larger than the semiconductor laser and photoelectric conversion efficiency is lower, causes problems in which power consumption is increased and in which stricter temperature control is needed. As a result, more restrictions are placed on the kind of light source that should be used in a compact projection-type display device.
Thus, recently, a method to obtain color light that is required has been provided by emitting light from a fluorescent body that uses light such as blue light or ultraviolet light of the semiconductor laser as excitation light.
Patent Literature 1 discloses a configuration where in a circular substrate attached to a wheel motor, a first region including a fluorescent body for emitting red light, a second region including a fluorescent body for emitting green light, and a third region for transmitting a blue laser beam are arranged in a divided manner. By rotating the wheel motor, red fluorescent light, green fluorescent light, and a blue laser beam are generated in time division to be used as light sources for a projection-display device.
The following effect is simultaneously provided: the fluorescent body is rotated by the wheel motor to disperse the energy of an excitation laser condensed on the fluorescent body, thereby preventing the fluorescent body from being thermally damaged.
Such a hybrid structure that combines a semiconductor laser and a fluorescent body is expected to be used as a light source for high-output compact projection-type display devices.
FIG. 1A is a perspective view schematically showing the general configuration example of a projection-type display device that uses a hybrid light source combining a semiconductor laser and a fluorescent body. FIG. 1B is a plan view schematically showing the general configuration example of the projection-type display device that includes the hybrid light source.
As shown in FIGS. 1A and 1B, light source device 1 included in the projection-type display device includes laser light sources 2a to 2c corresponding to respective primary color signals of R, G, and B, collimator lenses 3a to 3c, dichroic prism 4 as an optical member, circular substrate 6 coated with fluorescent body 5, wheel motor 7 for rotating circular substrate 6, optical integrator 8a for converting R, G, and B light combined by dichroic prism 4 into a rectangular light flux, spatial light modulation device 9a for modulating light from optical integrator 8a, and projection lens 10 for projecting light from spatial light modulation device 9a to a projection surface. Condenser lens 11a, mirror 12, and TIR (Total Internal Reflection) prism 13 are arranged between optical integrator 8a and spatial light modulation device 9a, and the rectangular light flux shaped by optical integrator 8a is guided to spatial light modulation device 9a. 
In this case, for easier understanding, a configuration using dichroic prism 4, DMD (Digital Micromirror Device) 14 as spatial light modulation device 9a, and a pair of fly-eye lenses 15a and 15b as optical integrator 8a is described. However, a configuration using a dichroic mirror or a cross dichroic prism as an optical member, a liquid crystal panel as spatial light modulation device 9a, and a rod-type integrator or a light tunnel including a transparent medium having a rectangular section as optical integrator 8a can also be used.
Next, referring to FIG. 1B, the projection operation of the projection-type display device will be described. As shown in FIG. 1B, laser beams 16a to 16c that are emitted from laser light sources 2a to 2c respectively corresponding to the three primary color lights of R, G, and B pass through collimator lenses 3a to 3c to enlarge beam diameters, and enter into dichroic prism 4 while being converted into parallel lights.
Dichroic prism 4, which is formed into a prismatic shape, includes a plurality of optical films formed on its inner surface to transmit light from fluorescent body 5 while reflecting light of a predetermined wavelength band from each of laser light sources 2a to 2c. Among the laser beams that enter dichroic prism 4, the color laser beams (laser beams 16b and 16c in the configuration example) that are directly used as light source lights are reflected on the optical films in dichroic prism 4 to exit from the other end side of dichroic prism 4.
On the other hand, the laser beam ((laser beam 16a in the configuration example) used for exciting fluorescent body 5 is similarly reflected on the optical film in dichroic prism 4 to be applied to circular substrate 6 coated with fluorescent body 5. At this time, fluorescent body 5 is excited by laser beam 16a while being rotated by wheel motor 7 to emit fluorescent light 17. Fluorescent light 17 enters into dichroic prism 4 again. Fluorescent light 17 exits from the other end side of dichroic prism 4 together with other laser beams 16b and 16c. At this time, laser beams 16b and 16c and fluorescent light 17 are combined to enter the pair of fly-eye lenses 15a and 15b, and are converted into a rectangular light flux having a uniform illuminance distribution. The rectangular light flux is then applied to DMD 14 via condenser lens 11a, mirror 13, and TIR prism 13, and subjected to light modulation according to an image signal. The rectangular light flux, which has been subjected to light modulation at DMD 14, passes through TIR prism 13 again to enter projection lens 10, and is magnified and projected to the projection surface from projection lens 10.
Coherent light that has high coherence, as in the case of the laser beam, has irregular patterns that are larger than the wavelength of the coherent light. For example, when the coherent light is applied to a rough surface such as a screen, the light is randomly scattered to generate spotty glistening bright and dark patterns called speckles. This is a random coherent phenomenon generated by superimposition of scattering lights of a single wavelength from respective points on the rough surface at each point of an observation surface.
Thus, in the projection-type display device using the laser light sources at a part or all of the R, G, and B light sources, when a video is projected to the projection surface of the screen, the laser beam is diffused on the projection surface of the screen to generate random noise (speckle noise) intensity in the light beam. In this case, when an observer watches the projected video on the screen to form a speckle image on the retina, the image is recognized as an unfocussed spotty glimmer. As a result, the observer feels uncomfortable or experiences fatigue, and the image that is viewed has undergone considerable quality deterioration.
In the projection-type display device that uses the laser beam as the light source, various methods for reducing such speckle noise have been provided.
In general, as a way to reduce speckle noise, there are two methods that include “achievement of incoherent laser beam” and “reduction of apparent speckle noise”.
The method of “achievement of incoherent laser beam” is a method for removing incoherence of the laser beam to convert it into incoherent light. “Broadening of wavelength width by high-frequency superimposition of laser beams”, “multiplexing of laser beam having delay larger than coherent distance”, or “superimposition of polarized lights orthogonal to each other” corresponds to this method.
These methods are essentially designed to prevent speckles from being generated by changing the property of light itself.
The method of “reduction of apparent speckle noise” is a method for reducing apparent speckle noise by superimposing and integrating image speckle patterns for a plurality of times in the time which is less than or equal to time (less than or equal to 20 ms) that cannot be recognized by a human eye in order to average speckle noises until such a point is reached at which the noise is outside the range of the human eye. “Swinging of screen” or “vibration of optical component” corresponds to this method. In these methods, speckles themselves are generated because the nature of light itself is not essentially changed. However, by an illusion that occurs in the brain, the speckles are prevented from being recognized by the human eye.
As in the case of the former, when the speckle noise is reduced by converting the laser beam into incoherent light, an element included in the semiconductor laser or a driving circuit must be directly retouched, or the optical system must be greatly changed. However, it is difficult to obtain satisfactory effects only by one method, and thus the method, that is combined with other methods and that is a multiple method, is likely to be used in many cases.
On other hand, in the latter case, when the apparent speckle noise is reduced, effects are conspicuous because an illusion that occurs in the brain. However, among these methods, the method based on screen swinging is only applied to a certain projection-type display device such as a rear projector because the mechanism becomes large and restriction that is related to the screen also occurs.
Among the latter methods of “reduction of apparent speckle noise”, “a method for reducing speckle noise by vibration of optical component” in particular will be described.
FIG. 2A is a perspective view showing a first configuration example according to a first related technology for reducing speckle noise. FIG. 2B is a perspective view showing a second configuration example according to the first related technology for reducing speckle noise. The first related technology is disclosed in Patent Literature 2. As shown in FIG. 2A, in the first configuration example according to the first related technology, a laser beam from laser light source 2d passes through collimator lens 3d, optical integrator 8b, and condenser lens 11b to be applied to spatial light modulation device 9d. In the first configuration example, a speckle pattern is temporally and spatially moved in an optical system by rotating optical integrator 8b including a pair of fly-eye lenses 15c and 15d around an optical axis. Accordingly, a speckle pattern formed on the retina of an observer is integrated to reduce apparent speckle noise.
As shown in FIG. 2B, in the second configuration example according to the first related technology, a laser beam from laser light source 2d passes through collimator lens 3d, condenser lens 24a, optical integrator 8c, and condenser lens 11b to be applied to spatial light modulation device 9d. In the second configuration example, similar effects are obtained by rotating, as optical integrator 8c, rod-type optical integrator 23a that is a transparent medium such as glass having a rectangular section around an optical axis as in the case of the first configuration example.
FIG. 3A is a plan view showing a first configuration example according to a second related technology for reducing speckle noise. FIG. 3B is a plan view showing a second configuration example according to the second related technology for reducing speckle noise. The second related technology is disclosed in Patent Literature 3. As shown in FIG. 3A, in the first configuration example according to the second related technology, diffusion plate 19f, that is a dynamic scattering medium rotated by motor 25, is disposed on an optical path between a pair of condenser lens 24b and collector lens 26a. As shown in FIG. 3B, in the second configuration example according to the second related technology, diffusion plate 19g, that is a dynamic scattering medium vibrated by transducer 27 of signal source 28, is disposed on an optical path between a pair of condenser lens 24c and collector lens 26b. With these configurations, because of the arrangement of diffusion plates 19f and 19g on the optical paths, scattering states on the optical paths are changed to temporally and spatially vibrate a speckle pattern. Accordingly, a speckle pattern formed on the retina of an observer is integrated to reduce apparent speckle noise.
FIG. 4A is a plan view showing a first configuration example according to a third related technology for reducing speckle noise. FIG. 4B is a plan view showing a second configuration example according to the third related technology for reducing speckle noise. The third related technology is disclosed in Patent Literature 4. As shown in FIG. 4A, in the first configuration example according to the third related technology, diffusion plate 19h is disposed on an optical path between beam magnifying optics 30 including a magnifying lens (collimator lens 3e) and collimator lens 3f and beam shaping optics 31 including a pair of fly-eye lenses 15e and 15f and condenser lenses 11c and 11d, through which a laser beam from laser light source 2e passes, and diffusion plate 19h is vibrated by means for applying motion 32a. Accordingly, by temporally and spatially vibrating a speckle pattern, a speckle pattern formed on the retina of an observer is integrated to reduce apparent speckle noise. As shown in FIG. 4B, in the second configuration example according to the third related technology, in addition to the first configuration example, diffusion plate 19i is also disposed between beam shaping optics 31 and spatial light modulation device 9d. By common or individual means for applying motion 32a and 32b, two diffusion plates 19h and 19i are vibrated, thereby enhancing the effect of reducing speckle noise.
FIG. 5A is a plan view showing a first configuration example according to a fourth related technology for reducing speckle noise. FIG. 5B is a plan view showing a second configuration example according to the fourth related technology for reducing speckle noise. The fourth related technology is disclosed in Patent Literature 5. As shown in FIG. 5A, in the first configuration example according to the fourth related technology, as in the case of the third related technology, diffusion plate 19j is disposed on an optical path between field lens 11f and spatial light modulation device 9e, through which a laser beam from laser light source 2f passes, and diffusion plate 19j is connected to diffusion plate swinging unit 36. In the first configuration example, a pair of fly-eye lenses 15g and 15h is used as optical integrator 8d. In the first configuration example, speckle noise is effectively reduced by setting a swinging speed V to satisfy “V>d×30” (displacement amount per second), where V (mm/s) is a swinging speed of diffusion plate 19j and d (mm) is a particle size of diffusion plate 19j. In addition, the loss of a certain amount of light of a laser beam caused by the diffusion plate is prevented by setting the diffusion angle of the diffusion plate to be limited based on a relationship between the numerical aperture of the illumination optical system and the brightness of the projection lens. As shown in FIG. 5B, in the second configuration example according to the fourth related technology, a case, where rod-type optical integrator 23b which is used in place of the pair of fly-eye lenses 15g and 15h and which is used as optical integrator 8e, is disclosed.